If a user tried to create a table with a generated column where
the generated column contained a JSON_TABLE, the server would
end up trying to access a null-pointer. The server already
rejects subqueries in generated column expression, but this check
could be omitted if a subquery was a subpart of a more complex
expression.

The MySQL manual says that "Subqueries, parameters, variables,
stored functions, and user-defined functions are not permitted.".
This patch fixes the bug by making pre_validate_value_generator_expr()
more aggressive so that expressions that contains either subqueries or
stored functions (either at root level or inside the expression in any
way) are rejected as generated column expressions.

Change-Id: I86870e66bc3b6220197f3519c721a7385c0d4cb5